1. Field of the Invention
The present invention relates to an internal combustion engine in which the valve characteristics of an inlet valve are made variable by the use of a variable valve train.
2. Description of the Related Art
A good startability is required for a reciprocating engine (internal combustion engine) which is mounted on a motor vehicle (a vehicle). In particular, a high startability is required for a cold engine in which fuel is difficult to vaporize.
On the other hand, there is an engine disclosed in JP-A-2003-129812 in which a variable valve train of a variable valve lift type is mounted on a cylinder head for variably controlling the valve characteristics of an inlet valve so as to match them with a running condition of the engine in which variable valve train a valve lift of the inlet valve is made to vary continuously from a maximum valve lift to a minimum valve lift based on a maximum lift center position with a valve opening period narrowed.
Specifically, according to the varying function of the variable valve train, as is indicated by chain double-dashed lines in FIG. 10, the valve lift of the inlet valve varies continuously towards the small valve lift based on the maximum lift center position thereof with the valve opening period narrowed.
Because of this, in determining a starter valve lift, a method is used in which initially, a fast idle valve lift suitable for fast idle when the engine is in a cold state, or specifically, a large valve lift α is determined which large valve lift α is determined in a wide valve opening period extending from a point near a top dead center to an area spreading further than a bottom dead center of an induction stroke period, and based on this large valve lift α, a starter valve lift β for use for a cold start is then determined. Namely, the valve lift amount is reduced based on the maximum lift center position from the valve lift α so as to set the small starter valve lift β. Specifically, to ensure an actual compression ratio, the small starter valve lift β is set by a lift curve in which a valve closing position of the inlet valve is disposed near the bottom dead center of the induction stroke period. Fuel is made easy to vaporize for a cold start of the engine by this lift curve (by a rise in temperature in the cylinder).
However, as is shown in FIG. 10, since the small starter valve lift β is obtained by reducing the fast idle valve lift based on the maximum lift center position by the use of the variability of the variable valve train, in the event that the valve closing position of the inlet valve is positioned in the vicinity of the bottom dead center of the induction stroke period, a valve opening position of the inlet valve delays largely from the top dead center of the induction stroke period or the valve lift amount of the inlet valve becomes somewhat too small.
When the valve opening position of the inlet valve delays, there is formed a blank period like a period m shown as extending from closure of the inlet valve to opening of the same in FIG. 10, that is, a period when a vacuum or depression is formed in the cylinder. Because of this, there is caused a fear that oil is caused to enter the cylinder from the circumference of the piston due to the depression so produced in the cylinder. In addition, the valve lift amount of the inlet valve which is slightly too small tends to easily call for insufficiency of intake air by volume, leaving many problems in the engine. In addition, although it is considered to advance overall the valve lift for elimination of the time period m in FIG. 10 so as to position the valve opening position in the vicinity of the top dead center, as this occurs, the valve closing position results in a point lying forwards of the bottom dead center, whereby there is a period when a depression is formed within the cylinder.
As a countermeasure against the problem above, there is proposed a technique for filling the period when the depression is formed within the cylinder by the use of a separate approach as is shown in FIG. 11 in which the phase of an exhaust valve is delayed by a variable valve train of a variable phase type and the valve opening position of the inlet valve is advanced by increasing the valve lift amount of the small valve lift β.
By the use of the technique so proposed, the formation of depression is suppressed in an ensured fashion, and the valve lift amount is increased. As is shown in FIG. 11, however, since the starter valve lift β is set in such a way that the valve lift amount is increased based on the maximum lift center position, the valve closing position is delayed largely to a point in the area spreading further than the bottom dead center of the induction stroke period. This then causes a fundamental problem that the actual compression ratio within the cylinder is reduced. Moreover, since a period when the exhaust valve is opened and the inlet valve is closed is formed in an area spreading further than the top dead center of the induction stroke period, a behavior is generated in which combustion gases are retaken in from an initial combustion on, and combustion tends to become unstable easily, resulting in a fear that a misfire may take place. Because of this, conversely, there is also caused a fear that the startability of the engine is reduced. On top of that, the delay of the phase of the exhaust valve delays the opening of the exhaust valve, this also leading to a problem that the effect is eliminated of increasing the temperature of a catalyst for purifying exhaust gases of the engine.
In addition to the variable valve train having the construction in which the valve lift of the inlet valve is made to vary continuously from the maximum lift to the minimum lift based on the maximum lift center while narrowing the valve opening period, there is also known a variable valve train having a construction in which the valve lift of an inlet valve is made to vary continuously from a maximum valve lift to a minimum valve lift while advancing the advance angle and further narrowing a valve opening period. However, with the variable valve train having the construction described above, although valve characteristics matching running or operating conditions of the engine can be obtained, it becomes difficult due to its variability that a valve lift for a fast idle of a cold engine is compatible with a valve lift for starting the cold engine.